OF THE FOSSIL PLANTS OF THE COAL-MEASURES. 
337 
le Stigmaria.” About the truth of these remarks there is no doubt, and seeing that 
the difference in their “ corps vasculaire, ” is, as I have shown in the case of the 
Burntisland plant, merely one of age and growth, I marvel that M. Grand-’Eury 
does not discern whither his own admissions lead him. 
I will now call attention to some further details in the organization of Lepido- 
dendron selaginoides, which approaches the nearest to the type of L. Harcourtii of 
any of the Lepidodendra. This is the plant which Mr. Binney designates Sigillaria 
vascularis. I presume that he does so because it possesses a thin exogenous vascular 
zone. If so, he merely reasons in a circle, since, with this exception it does not 
possess a solitary feature connecting it with the Sigillarise ; it does not display the 
faintest trace of the longitu di nal ridges and furrows so characteristic of the true 
Sigillarise, whilst, as I have already shown,* its bark and leaves display every 
feature that characterizes a true Lepidodendron, a conclusion in which I am sup- 
ported by the opinion of Mr. Carruthers.! I am indebted to Mr. James Spencer, 
of Akroydon, near Halifax, for the specimens from which I have obtained the 
fine additional sections now figured. 
It is not necessary to enter again into a detailed description of the structures 
representing the medullary axis of the plant, in which axis cells and vessels are more 
or less intermingled in a way that is essentially Lycopodiaceous. I have already 
done that in my second memoir just quoted, and later observations have suggested 
nothing that requires either to be added to or retracted from my previous descriptions. 
Fig. 33 is a portion of a transverse section of a stem in which c represents some of 
the peripheral vessels of the medullary group, which group represent the primary 
vascular bundle of a young axis. That these vessels increase with age up to a 
certain point, both in size and number, is unquestioned ; but I am still as far as ever 
from learning where and by what agency the increase in number has been effected. 
At d we have part of the exogenous zone, composed of radiating laminae (e) of vessels 
that increase in size from within outwards, and which laminae are separated from each 
other at very frequent intervals by the medullary rays, f The innermost cortical 
layer is seen at g, g , g", the latter being a narrow band bridging over the space so con- 
stantly left vacant, and showing that, when occupied by its original tissues, it consists of 
parenchyma like that seen at g. At m, m, are two vascular bundles passing outwards 
through the bark. Nothing can be more clear than that, whatever may be the mode 
of increase in the vessels constituting the cylinder c, and which represents the 
vascular cylinder of Lepidodendron Harcourtii, in the outer zone, d, we have a 
definite example of exogenous growth, but one in which, unlike the Diploxyloid forms 
from Burntisland and elsewhere, only undergoes a very limited development. At m 
we find one of the foliar bundles of small vessels issuing directly from the inner or 
medullary series of vessels, c, and passing outwards through an opening in the 
* Phil. Trans., 1872, Plate XXIV., figs. 1, 5, and 6. 
t ‘ Monthly Microscopical Journal,’ Oct. 1, 1869, p. 179. 
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